T9 allows you to enter entire words by a single key press on a letter, instead of having to tap a key multiple times to rotate through all the possible letters until you get to the one you want.

Predictive Text Entry, (also know by the most popular type, T9,) allows you to enter text by pressing only one key per letter on a keypad where multiple letters share keys. As you enter a word ...

Well, so did [Guy Dupont], so he created a USB keypad with T9 predictive text built-in to turn typing into a one-handed affair. Video after the break. T9 was the first predictive text technology ...